Jobcentre staff around the country have been involved in a drive to kick people off benefits amid pressure to meet welfare targets set by their managers, the government has admitted.

The Department of Work and Pensions (DWP) initially dismissed revelations in the Guardian last weekend that Jobcentre Plus employees were tricking vulnerable claimants into losing welfare entitlements. A whistleblower said staff at his jobcentre were given targets of three people a week to refer for sanctions, where benefits are removed for up to six months.

The work and pensions secretary, Iain Duncan Smith, appeared on Sky News on Sunday claiming it was "claptrap" that anyone would "hand out edicts to staff to sanction three people", and said the story was a "conspiracy".

Since then, further email evidence has been uncovered showing individual or group targets are being imposed to stop people's benefits at offices across the country. In some cases staff have claimed they have been threatened with sanctions themselves if they do not reach the targets.

Following the Guardian investigation, a group of trade unions representing jobcentre staff have written to management seeking "urgent clarification" on the issue of targets for referrals and sanctions.

The DWP has backtracked and released a statement confirming the practice had been going on in some offices due to a misunderstanding between the department and some jobcentre managers. It insisted this was no longer the case.

"A few weeks ago ministers discovered that their message to be clearer about conditionality had been misinterpreted by a small number of Jobcentre Plus offices who had imposed targets for the number of sanction referrals. These targets were immediately removed.

"We are clear that there is no wrong or right level of how many sanctions an office should make and they should only be made where people have not adhered to their jobseeker obligations. We have already taken rapid steps to reinforce this message to our staff. Ministers would not countenance any target for sanctioning customers."

The Guardian has spoken to several more jobcentre staff who, speaking anonymously, claim that targets and pressure to stop people's benefits still exist in their office, and that vulnerable clients are often affected. One employee claimed the practice had been going on at his office since they joined in July 2009.

One personal adviser backed up claims that targets led advisers to "set up" claimants. He said: "On the question of tricking customers – it's quite true. You box them in so that they go wrong themselves. The whole system is now orientated to stopping rather than enabling."

He said people with poor English skills were often targeted in his office. "For example, an African man who had managed to get part-time work and was studying English. His jobsearch was far more adequate than most, but managers specifically spent time going through it and comparing it to his agreement to see where they could trip him up. It was deemed inadequate and he was sanctioned. It's easy to sanction these people because he didn't know what was going on."

Another jobcentre employee with several years' experience said: "If staff are chasing targets, they will themselves target the easiest [claimants], for example people with learning disabilities, or people with English as a second language. It's the easiest way to meet those targets under pressure."

He said he had seen people threatened with the sack for not meeting targets. "If you are a good adviser, you would actually expect to sanction less people, as you will convince them of the importance of meeting the conditions to get their jobseeker's allowance [JSA]."

Another former personal adviser said the priority was clear when he joined in July 2009. "The first thing that happened is they took us to a presentation where we were shown a big league table of statistics, including sanctions. They pointed out the offices that were doing well – it's like it's a big competition.

"I was threatened by management for asking too many questions. I felt what we were doing in some cases was unlawful." He said he believed offices had "their own take" on social security law in terms of the strictness with which they were sanctioning people, and that "management, and the culture of [Jobcentre Plus] – with only a few exceptions – viewed claimants with contempt."

A jobcentre personal adviser agreed that targets for sanction referrals were not new, but said: "The targets [for sanctions] have got higher and the options for getting people into jobs have got fewer. People are being treated as numbers, there is no sense of individuality.

"The advisers in our office are struggling to get through the huge numbers of claimants, so no wonder [the claimants] are being treated like numbers … We are pushing people through as fast as possible."

It has also emerged that Citizens Advice in Wigan, one of the areas investigated by the Guardian, had compiled a report on the "growing problem in the way the jobcentre are dealing with the issue of sanctions", and had submitted it to the DWP.

The report says 41 people came to Citizens Advice with the problem between September 2010 and January this year, compared to eight people in the same period the previous year.

"We feel that many of our clients who are genuinely doing all they can to improve their chances of seeking employment and are meeting their jobseeker's agreements in full, are being unfairly sanctioned," the report said. "It could be argued that the way some JSA sanctions are applied is vindictive."

Cases documented include a homeless man and clients whose homes were at risk due to the knock-on effect on other benefits. The report says many of the people sanctioned often did not realise until they checked their bank accounts, and that some clients had been "actively advised against appealing [against] their decisions" by jobcentre staff.

A jobseeker who signed on for the first time on Monday told the Guardian: "I did not expect the conversation I had with the jobcentre adviser, who was completely unknown to me. He seemed very afraid, that if he didn't carry out these instructions he would be the next one signing on.

"He asked me to imagine: 'A manager comes round on a Friday afternoon and says I have to find one more person to sanction to meet my target; I have one client with previous for GBH, and another with learning disabilities. Who am I going to choose?'"

The Public and Commercial Services Union general secretary, Mark Serwotka, called on Iain Duncan Smith to apologise for "outrageous spin". "When this story broke Mr Duncan Smith described it as claptrap and a conspiracy, but faced with the overwhelming evidence that these targets are still in place, his department has been forced to backtrack.

"The secretary of state should issue a full and public apology. When MPs return from their Easter break he should also be asked to explain to parliament why he tried to mislead the public.

"We fundamentally oppose the use of targets for welfare sanctions and we call on the Department for Work and Pensions to put an immediate stop to this abuse of the system."